Processes for forming of boride layers on metallic surfaces are known in the art. Boriding takes place by diffusion of boron atoms into a metallic material which has the ability to form boride compounds such as MeB, Me2B or MexBy. The so formed boride layer improves the hardness, abrasion, corrosion, oxidation, and fatigue resistance of the materials.
Boriding can be conducted with different processes by using paste, liquid or gas phase boron source (amorphous or crystalline boron, ferroboron, boron carbide, borax, boric acid, NaBF4 and other boron compounds) and a reducing agent (SiC, FeSi, B4C) at temperatures between 700-1000° C. within a time interval varying between 1-10 hours.
A process that can be used for boriding is electrochemical boriding that is conducted in molten salt electrolytes at temperatures ranging between 700-1100° C. For this process, boron containing electrolytes are used and reduction to boron is achieved through the electrolytic process. However, in these electrolytes generally toxic compounds such as fluorides are needed in order to obtain an acceptable rate of boriding.
Another type of method for forming boride layers on the material surface is conducted through the coating processes such as PVD and CVD. In PVD techniques, a boride material is used as the target material (e.g. TiB2) which may be difficult to process and also expensive. The coatings produced with these methods are having problems of adhesion and high brittleness. In CVD processes boron source is a boron containing gas (such as BCl3) which are highly toxic and expensive. In these cases the boride compound is formed as a separate layer.
A classical electrochemical boriding process is conducted in the presence of activating agents such as alkali fluorides for electrochemically activating the boron compounds. A borided layer thickness of 130 μm is obtained on ferrous metals and alloys within 4-8 hours under the standard heating conditions according to the ASTM Metal Handbook. Decreasing the process duration without concession of the desired coating thickness will obviously provide considerable advantage over the known methods. A further advantage will appear if use of said fluoride compounds is eliminated in the electrolyte solution.
U. Fastner et al., “Electrochemical deposition of TiB2 in high temperature molten salts”, J. Alloys Compd., 2007 provides information with regard to such electrolytes having fluorine content (i.e. NaCl—KCl—NaF—KBF4—K2TiF6). The publication reports that performance of the electrolyte is significantly reduced with the thermal decomposition of the tetrafluoroborates. Further, electrolytes containing for instance LiF—NaF—KF are disclosed by Jun et al., “Preparation of highly preferred orientation TiB2 coatings”, RARE METALS, vol. 25, No. 2, p. 111, 2006 wherein effect of the current frequency and density on the formation of the TiB2 coating is evaluated. An overall review of the electrolyte solutions having fluoride content for electrochemically activating the boron species has been reported by V. Danek et al., “Thermodynamic and structural aspects of electrochemical deposition of metals and binary compounds in molten salts”, Coordination Chemistry Revies, 1997. The non-patent literature aims to improve the electrolyte performance in general, however none of the cited documents suggests eliminating use of said fluorinated compounds and the problems associated thereto.
In the presence of the fluorinated compounds as per addressed above, performance of the electrolyte significantly decreases particularly in boriding processes because of the thermal decomposition, and boron-fluoride or alkali-fluoride gas emission. On the other hand, omitting said compounds from the electrolyte solutions reduces electrochemical activation of boron which leads low yielded boron diffusion. Any person in the field would also appreciate the drawbacks of said gas emissions in terms of the environmental aspects. The present invention eliminates use of said fluorinated activator compounds by means of conducting electrochemical boriding in a high frequency induction heating system and providing a unique electrolyte composition.
JP 57161090 discloses a method for boriding a structural material wherein an abrasion wear resistant surface is obtained by coating one or two of the elements selected from IVA, VA and VIA groups. Na2B4O7 is used as the boron source and DC electrolysis is applied in the electro-deposition medium. The method bears the standard drawbacks in terms of process duration as exemplified above with reference to the ASTM Metal Handbook. The method does not refer to any solution for above problems, and does not mention about high frequency induction heating of the electrolytic salt medium which may accelerate the boron diffusion as in the present invention.
GB 1,422,859 discloses a similar process for hardening a metal or alloy work, the process comprising the steps of: a) “covering” the metal work with a molten layer of a treating material selected from boron oxide and borates, and optionally elements of groups IVB, VB and VIB (according to the CAS-system); b) transferring heat to said molten layer to maintain the same at a temperature from 700° C. to 1200° C.; c) introducing an inert anode into said molten layer and passing electric current at a current density of at least 0.5 A/cm2 through said work and molten layer until the surface region of said work is permeated with boron with or without said elements. Apparently, boron diffusion starts in step c) after passing an electric current for electrolyze. Heating means (e.g. high frequency induction) are needed to be used solely in the process steps a) and b) for better “covering” with the molten treating material. However, the disclosure does not accept supplemental heating as a requirement for boron diffusion in the case of electrolysis of step c) as understood from the description (page 4, lines 101-103). This is because great amount of heat is produced during electrolysis which renders an additional heat supply unnecessary. According to the explanations heat is supplied only for the purpose of heating the medium in steps a) and b) but not for increasing the boron diffusion. In other words, the document is silent about the surprising effect provided with high frequency induction heating during boron diffusion, heating means are used solely for heating purposes, and the disclosure even excludes the use of such heating means during electrolysis perceiving it as unnecessary.
U.S. Pat. No. 3,824,134 discloses a method for producing hard metallic boride layers comprising carburizing of a ferrous metal substrate and then coating this surface with a metal that has high affinity for carbon (e.g. Cr). This layer forms an impervious carbide layer during heating thus eliminating diffusion of iron into this layer during boriding. Hence, this study mainly aims to eliminate ferrous boride formation within the boride layer. Eventually, the patent disclosure deals with different problems other than those of the present invention.
DE 1 796 215 discloses boriding of metals, particularly boriding of steel, using ferroboron, amorphous boron, boron carbide or borax as boron precursors, with fluoroborate as an activator agent. Despite the description addresses the use of high frequency induction heating for boriding methods conducted in aqueous solutions as well as in the paste boriding treatments, disclosure is still silent for providing solutions to the problems posed in the present invention. First of all, the procedure is mainly designed for boriding of ferrous metals or alloys, and does not bring solutions associated with boronization of non-ferrous metallic structures. On the other hand, using of induction heating regime in the mentioned type aqueous solutions (not in the molten salts) indicates that such heating regime is solely used for heating of metals to high temperatures and this is given only for general information. The information regarding use of a high frequency inductor in a paste boriding procedure is technically irrelevant with the electrolytic processes conducted in molten salts. Disadvantage of this process is that particles from boriding agent are welded on the metal surface rendering the surface characteristics very poor, hence high frequency induction heating becomes unsuitable for this process (Page 3, second paragraph). Furthermore, the process disclosed in the specification strictly requires use of the halogenated compounds (I.e. KBF4) which are eliminated with the process disclosed in the present invention.
In relation to the above findings, there is still need for an improved method of electrochemical boriding in order to overcome these problems. These drawbacks of the known methods are eliminated by the electrochemical process of claim 1.